1. Technical Field
The invention is related to streaming media rendering, and more particularly to a system and process for obtaining progressively higher quality versions of an audio and/or video program over a client-server based network.
2. Background Art
Audio and video information is commonly sent from a server to a client over a network connection, particularly over the Internet. For example, many news and sports web sites on the Internet contain short video clips which can be accessed by a user. One typical way that this happens is for a user to download the data associated with the desired clip. This is accomplished by a client computer associated with the user making a request for the data from a server upon which the data is resident. The server then transfers the requested data to the client via the network. Once all the data has been received by the client, the client computer renders it and presents it to the user in the normal manner. However, when data is transferred over a network, and particularly over the Internet, the channels between the server and client can vary dramatically in capacity, often by two or three orders of magnitude. These differences in capacity exist because the data transmission rates associated with the connections to a particular client can vary (e.g., phone line capacity, LAN and/or modem speeds). This heterogeneity in capacity can cause problems, particularly if high quality audio and video is desired. For example, downloading a high quality, and therefore large bandwidth, version of an video clip from a website on the Internet could mean waiting for much longer than the duration of the clip itself. Thus, the user has to wait to see the video clip, often with the result of frustrating the user. Furthermore, the user may not know if the video clip is of interest without viewing it, so waiting to download something that may not even be interesting is doubly unattractive.
The downloading issue can be avoided by using a form of audio and video data transfer referred to as a real-time unicast multimedia presentation. Essentially, this scheme involves streaming data associated with a requested video program from the server to the client over the network. As the data is received by the client, it is rendered and presented to the user on a nearly real time basis. However, the aforementioned bandwidth limitations typical of a network, and particularly the Internet, also create problems for this type of transfer. For example, the typical bandwidth available on a network like the Internet is inadequate to allow the streaming of a high quality color video. Thus, a particular client may not have the bandwidth available to receive the highest quality transmission that a server is capable of providing.
To overcome this bandwidth problem, audio and video information can be transmitted via a layered scheme. In a layered scheme, audio and video information is encoded in layers of importance. Each of these layers is transmitted in a separate data stream, which are in essence a sequence of packets. The base layer is an information stream that contains the minimal amount of information, for the least acceptable quality. Subsequent layers enhance the previous layers, but do not repeat the data contained in a lower layer. In order to obtain the higher quality, a client must receive the lower layers in addition to the higher layers that provide the desired quality. Thus, the layers are hierarchical in that there is at least one base layer, and one or more additional higher level enhancement layers. There can in fact be several hierarchical layers building up from a base layer with each subsequent layer being dependent on the data of one or more lower level layers and enhancing those lower level layers. An illustrative (but perhaps not particularly realistic) example of a layered video program would include a base layer that consists of black and white video of every odd numbered video frame, a second layer that consists of black and white video of every even numbered video frame, and a third layer that consists of color information for all frames. Playing only the first layer would get a black and white video at ½ frame rate (i.e., somewhat jerky).
Playing the first and second layers together would yield a black and white video at full frame rate (smooth motion). Playing all three layers would yield a color video at full frame rate.
In a layered scheme, a client can request as many layers as desired, provided the total bandwidth of the layers is not greater than the bandwidth available on the network. For example, if the client is connected to the Internet by a 28.8 Kbps modem, then it can feasibly subscribe to one, two, or three 8 Kbps video layers. If it subscribes to more than three such layers, then congestion will certainly result and many packets will be dropped randomly, resulting in poor video quality. By observing packet drops the maximum number of layers that can be supported can be determined.